The majority of bidirectional sensors already existing use permanent magnets associated with a more or less complex magnetic circuit, made from ferromagnetic material used to guide and/or concentrate the magnetic flux generated by the permanent magnet or magnets, but to the detriment of the cost and performance of the sensor. Thus, in the prior art, the patent FR 2786266 of the applicant is known, relating to a position sensor in two directions but in which the space requirement and the surface area of the magnet used limit the practical use of this sensor for long travels. This sensor also has high hysteresis due to the ferromagnetic stators and the measurement depends on the variation in the remenant induction, which must therefore be compensated for.
Moreover, European patent EP 800055 describes a linear angular position sensor. This sensor delivers analogue signals that are difficult to use since they are non-linear and of a low level. Such sensors require several separate measuring points for measuring the relative position in two directions. In addition, they require stator parts that contribute hysteresis and the sensitive elements measure the amplitude of the field and are therefore sensitive to geometric tolerances and temperature. The U.S. Pat. No. 4,639,667 or WO 9716736 describe sensors functioning according to principles that do not make it possible to deliver linear and independent signals representing the position in two dimensions.
There also exist bidirectional sensors that are merely the putting end to end of two independent unidirectional sensors, such as for example the patent WO 2008138662 and the U.S. Pat. No. 6,175,233 describing two linear sensors that each measure one direction. For each sensor there is a magnet and an element for detecting the magnetic field, the consequence of which is to lead to a high space requirement and high production cost. In addition, these sensors measure the amplitude of the field and are therefore also sensitive to geometric tolerances and temperature.
There are also known, in the prior art, the U.S. Pat. No. 7,421,923 and U.S. Pat. No. 7,293,480, which are sensors for detecting gears engaged by a gear lever. Such patents present a solution for detecting positions in two directions but use a magnet and at least as many Hall sensors positioned in space as there are gears to detect. It is therefore necessary to use an array of sensors for discriminating single positions and to obtain a digital detection of the gears. The multiple number of sensors means that this solution is expensive to implement and does not offer means for knowing the intermediate positions.
To remedy the problems relating to the position detection by measuring amplitude described above, there exist position sensors that measure the rotation of the magnetic field, in other words the direction thereof, rather than the amplitude thereof. However, this applies mainly to unidirectional rather than bidirectional sensors.
There are known for example in the prior art sensors as described in the patents FR 2898189 and FR 2909170 of the applicant, which use the direction of the field rather than the amplitude for detecting a relative position between a magnet and a magnetosensitive probe. This measurement of direction makes it possible to be insensitive to temperature and to mechanical clearances and does not use any ferromagnetic part and therefore does not have magnetic hysteresis. However, such sensors measure only one magnetic field direction via the calculation of a single amplitude ratio from two components of the magnetic field, and can therefore know the relative position of a movable magnet with respect to a magnetosensitive probe only in one direction rather than two. Likewise, the patents and patent applications U.S. Pat. Nos. 6,731,108, 6,960,974 and WO 2004015375 afford only measurement of the linear movement of a magnet with respect to one or more magnetosensitive elements using the field direction. However, for practical implementation of travels greater than 20-25 mm, these sensors require several probes placed on the various parts of the travel, which increases the cost of the sensor and requires precise positioning of the probes.
Solutions are however known in the prior art for measuring bidirectional position and using the measurement of the rotation rather than of the amplitude of the magnetic field but, in the case of very specific applications to control levers (joysticks). Thus the patent applications US 2007024043 or US 20090062064 describe sensors for joysticks that comprise a simple magnet magnetised unidirectionally, along its thickness, and a probe that measures only two components of the field and therefore a single field direction (the angle formed by the two components). This principle does not make it possible to deliver independent linear signals in two directions. Systems of the joystick type are in addition limited only to rotations and cannot measure translations. Moreover, the angle that can be detected by such a joystick system is limited to around 30 degrees. Beyond this, the magnet is situated very far away from the probe, which no longer sees enough magnetic field to deduce a position therefrom. In fact, for a practical implementation for travels greater than 40 degrees, these sensors require several probes based on the different parts of the travel, which increases the cost of the sensor.
There is also found in the prior art a Melexis application note for a measurement of two rotation angles (http://ww.melexis.com/Sensor ICs Hall effect/Triaxis Hall ICs/MLX90333 648.aspx) where two joystick configurations are presented. The first is a solution where the centre of rotation of the bipolar magnet is merged with the measuring point, which requires a complex and bulky mechanical system that cannot easily be integrated in an application. The second configuration presents a solution where the centre of rotation of the magnet is situated behind the magnet (the magnet is between its centre of rotation and the sensitive elements). In this case, the three components of the magnetic field are used to determine two rotation angles. The magnet used is a cylinder with a very small diameter with a magnetisation that is constant in amplitude and direction along its thickness. That is to say the magnetisation of the magnet at any point on this magnet has the same modulus and is perpendicular to the top and bottom faces of the magnet. This very specific configuration is intended only for measuring two angles and for very short travel (around 30 degrees). This is because, in order to be able to detect the rotation of the magnet with the algorithm used, it is necessary for the diameter of the magnet to be small (theoretically a punctiform magnet with radial magnetisation), which means that, as soon as we have a small rotation of the magnet, the magnet moves away from the magnetosensitive elements and the magnetic induction becomes too small at the magnetosensitive elements of the probe to have precise detection of the rotation of the magnet. This is why this type of system requires a magnet with very high remanence (typically Br>1.2 T) and very thick (thickness >10 mm), which is therefore expensive and difficult to magnetise, with what is in addition a large axial thickness (typically >10 mm), which causes a problem of space requirement. In addition, with these solutions, the mechanical air gap between the surface of the flat magnet and the measuring point varies according to the rotations of the magnet, which involves deterioration in linearity and a larger air gap than necessary to avoid collision of the edges of the magnet with the probe support. The ideal thing for preventing this is a magnet with a very small diameter but which poses the problems already mentioned above.